In line with recent advancements in optical technology, various display technologies replacing a conventional cathode ray tube (CRT), such as a plasma display panel (PDP), a liquid crystal display (LCD), and an organic electroluminescent display (OELD), have been developed and have become commercially available. Meanwhile, various polymer films such as a polarizing film, a polarizer protective film and a retardation film, as well as a light guide plate and a plastic substrate, have been used for such display devices and there is a trend for the use of such polymer materials in a display device of which required characteristics have become highly advanced.
Currently, the most widely used polymer film for a display is a triacetyl cellulose (TAC) film which is used for a polarizing plate protective film or the like. However, the TAC film may have limitations in that the polarizability thereof may be degraded, a polarizer and the film may be separated or optical properties thereof may deteriorate when the TAC film is used over a prolonged period of time in a high-temperature or high-humidity conditions. In order to resolve the foregoing limitations, a polystyrene-based polymer film or an acryl-based polymer film, such as methyl methacrylate or a polycarbonate-based polymer film, have been suggested as alternatives to the TAC film. The foregoing polymer films may have excellent heat resistance. However, birefringence may be generated during film alignment, thereby adversely affecting the optical properties thereof, because the polystyrene or polycarbonate film has an aromatic ring in the polymer, and with respect to the methyl methacrylate, a retardation value thereof is relatively small in comparison to polystyrene or polycarbonate, but the methyl methacrylate is insufficient to be used as a material for an optical device such as a liquid crystal device requiring a high level of precision.
In order to address such limitations, a method of copolymerizing or blending a monomer or a polymer having positive birefringence with a monomer or a polymer having negative birefringence has been suggested for a material for a polymer film having a low retardation value, as well as excellent heat resistance. A typical material used according to the foregoing method may be a copolymer of benzyl methacrylate and methyl methacrylate. However, with respect to the copolymer of benzyl methacrylate and methyl methacrylate, there is a limitation in that heat resistance is insufficient.
Meanwhile, as a method of improving heat resistance, a three-component copolymer composition including benzyl methacrylate, methyl methacrylate, and methacrylic acid has been suggested. However, with respect to the three-component copolymer including benzyl methacrylate, methyl methacrylate, and methacrylic acid, a retardation value and optical properties thereof are excellent, but there is a limitation in that a curling phenomenon may be generated, in which a polarizing plate is severely bent or distorted when the copolymer is laminated with a polarizer and a TAC film to be used, because a thermal expansion coefficient of the three-component copolymer may be higher than that of the TAC film used for a polarizing plate protective film. When the foregoing curling phenomenon occurs in a polarizing plate, display quality may deteriorate due to the occurrence of a light-leakage phenomenon in the polarizing plate and liquid crystals in a display panel may also be damaged. Therefore, urgent improvements to rectify the foregoing limitations are required.